1. Field of the Invention
This invention relates to bimetallic supported olefin polymerization catalyst systems, methods of producing such catalysts and methods of polymerizing alpha-olefins with such catalysts to form polymers having multimodal or broad molecular weight distribution. More particularly, this invention relates to the preparation of a catalyst component comprising a metallocene-containing transition metal compound, a non-metallocene-containing transition metal compound, a magnesium containing compound and a polymeric material. The catalyst component is combined with a cocatalyst compound to produce a catalyst composition.
2. Description of the Prior Art
A number of methods exist for producing polymers possessing multi-modal and/or broad molecular weight distribution. U.S. Pat. No. 4,414,369 to Kuroda et al., U.S. Pat. No. 4,420,592 to Kato et al. and U.S. Pat. No. 4,703,094 to Raufaut describe processes employing two reactors operating in series, operating at significantly different hydrogen concentrations and thus producing polymer containing both high and low molecular weight polymer. Cozewith et al. in WO 86/03756 describe a melt blending technique in an extruder of polyolefins. However, these methods are an expensive, cumbersome and time consuming procedure of producing multi-modal molecular weight distribution polymer.
Alternatively, a single reactor can be used for the production of broad or multi-modal molecular weight distribution polymer by using blends of catalyst with different propagation and termination rate constants for ethylene polymerization. U.S. Pat. No. 4,530,914 describes the use of a homogeneous catalyst system consisting of two different metallocenes for the production of broad molecular weight distribution and/or multi-modal molecular weight distribution polyolefin. U.S. Pat. Nos. 4,937,299 and 4,522,982 describe using a homogeneous catalyst system containing two or more metallocenes to produce reactor blends. In further developments, U.S. Pat. Nos. 5,070,055 and 5,183,867 describe silica supported catalysts containing two different active centers.
Despite the catalyst improvements described in the aforementioned patents, the preparation of supported bimetallic catalysts is complicated and expensive, for example, requiring intermediate introduction of expensive and troublesome aluminoxanes in catalyst preparation. For example, U.S. Pat. No. 5,183,867 requires introduction of aluminoxane into the support. Any aluminum containing co-catalyst used is in addition to aluminum compound in the support. Furthermore, procedures typically used for the preparation of suitable silica supports such as spray drying or re-crystallization processes are complicated and expensive. Also, high calcination temperatures are required to remove water, which is a common catalyst poison. These steps represent a significant proportion of the preparation of the catalyst. Moreover, the use of silica as a support results in support material remaining in the polyolefin product, e.g., high ash and metal residues, which can affect product processing or product properties, such as optical properties.
In some instances, polymer supports have been used in olefin polymerization catalysts. U.S. Pat. No. 5,707,914 describes a catalyst component which includes polymer carrier, organic zirconium or hafnium compound, an organic compound containing Group I-III metal such as organoaluminum, and a cyclic organic compound containing two or more conjugated double bonds. This latter cyclic compound component has no transition metal. U.S. Pat. No. 5,707,914 describes the synthesis of a supported metallocene catatylst prepared in situ, with a formulation that consists of one active metal site (monometallic) mainly zirconium, used to produce polyethylene resin with narrow, possibly medium, molecular weight distribution. In contrast, the present invention provides a different catalyst capable of producing polyethylene with broad and bimodal molecular weight distribution. Moreover, the catalyst of the present invention has much higher activity.
An object of the present invention is to provide catalysts which overcome the difficulties encountered in the prior art.
The present invention provides a catalyst component comprising a transition metal-containing metallocene compound (referred to as the metallocene compound hereafter), a non-cyclopentadienyl transition metal compound (referred to as the transition metal compound hereafter), a magnesium compound and a polymeric material which acts as a support. The catalyst component, when used in conjunction with co-catalyst such as an organoaluminum compound or a mixture of organoaluminum compounds, can be used for ethylene polymerization to produce linear low, medium and high density polyethylenes and copolymerization of ethylene with alpha-olefins having about 3 to about 18 carbon atoms. Furthermore, the catalyst system has the ability to produce polymers with a varied range of molecular weight distributions, i.e., from narrow molecular weight distribution to a broad molecular weight distribution and/or multi modal molecular weight distribution.
In a preferred embodiment, the catalyst composition is prepared by a process comprising combining polymer particles, magnesium compound, metallocene compound, and transition metal compound to provide a solid catalyst component, and combining the solid catalyst component with a cocatalyst compound to provide a polyolefin polymerization catalyst composition.
The invention also includes processes of making and using the catalyst composition.